Methods of the kind which is described above are known. With such methods, cladding tubes of Zr-based alloys which contain Nb are thus produced. For example, U.S. Pat. No. 5,648,995 describes such a method and a cladding tube of this kind.
When a cladding tube is used in a nuclear reactor, it contains nuclear fuel, usually in the form of pellets comprising enriched uranium, usually in the form of UO2. The cladding tube with its content thus constitutes a fuel rod. Because of the very particular environment in which cladding tubes are used, different requirements must be fulfilled.
There are mainly two kinds of modern light water reactors: boiling water reactors (BWR) and pressure water reactors (PWR). In these kinds of reactors different conditions prevail which call for different requirements on the parts which form part of the reactors. In a PWR, the fuel rods are cooled mainly by water that is in a liquid phase under high pressure. In a BWR, the pressure is lower and the water which cools the fuel rods is evaporated such that the fuel rods are surrounded both by water in a liquid phase and in a steam phase. Furthermore, the fuel assemblies have different construction in a BWR and a PWR. In one kind of BWR, the fuel rods in a fuel assembly extend all the way between a top plate and a bottom plate which keep the fuel assembly together. In a PWR, on the other hand, the fuel rods are usually held in position with the help of spacers and do not reach all the way to the top plate and the bottom plate.
A fuel rod which is used in a nuclear reactor is exposed to high temperatures and pressures. Over time thereby creep phenomena occur. Such a creep should as far as possible be avoided since it can have negative effects. For example, a creep of the fuel rods may have as a consequence that they will press against the fuel pellets which are located therein. The neutron radiation to which a fuel rod is exposed when it is used may also have as a consequence that the fuel rod tends to grow with time. Also such a growth caused by neutron radiation may have undesired effects. It should therefore be avoided that the cladding tube grows to a larger extent. Modern fuel rods which are produced in suitable zirconium alloys and which undergo special heat treatments during the production often have a relatively low tendency to grow when they are exposed to neutron radiation. The tendency to grow may be reduced, inter alia in that the cladding tube during the production undergoes a final recrystallization anneal.
Through a suitable choice of the material for the cladding tube and a suitable method of production, the cladding tube can obtain suitable properties concerning for example hardness and ductility.
In the environment where the cladding tubes are used they may be subject to different corrosive attacks. These attacks may come from the outside or from the inside. The attacks from the inside often have their basis in an influence from the nuclear fuel material that is located there, so-called pellet-cladding interaction (PCI). If a crack is formed through the cladding tube (a so-called primary damage), water may penetrate in through the crack and spread along the inside of the tube. This may lead to new corrosive attacks from the inside of the tube, so-called secondary damages. A cladding tube of zirconium or zirconium-based alloys may also react with hydrogen such that hydrides are formed in the cladding tube. These hydrides may be formed from the inside of the tube, particularly if a crack has been formed such that water has penetrated into the tube. These hydrides make the tube more fragile and the probability for the formation of cracks increases. Particularly hydrides that extend in a radial direction through the tube constitute an increased risk for crack formation. Such radial hydrides may therefore speed up possible secondary damages and crack formations.
The complicated chemical, mechanical and metallurgical conditions that are the case in a nuclear reactor have lead to the fact that a very large number of suggestions have been proposed for the selection of materials and for the methods of production of cladding tubes. Even small changes in the composition of alloys or production parameters may have a large importance for the properties of the cladding tube.